For the Visual Studio C compiler we need to annotate our public symbols
with `__declspec(dllimport)` to ensure they are visible when dynamically
linking to Epoxy's DLL.
This is needed because under Windows we use a dispatch table, instead of
wrapper functions, thus the symbol visibility rules change. Compiling
with MingW will automatically add `__declspec(dllimport)` for us.
Thanks to Nirbheek Chauhan for the help in debugging the issue.
Fixes#104